What is the Custom Extensions Module used for?

The Custom Extensions module provides you with a facility to register any custom extensions or feature codes that you have created in a custom script or dialplan when the PBX doesn't otherwise know about them. This makes the Extension Registry aware of your own extensions so it can detect conflicts or report information about your custom extensions to other modules. Also, this helps prevent you from creating a duplicate extension number somewhere else in the PBX.

How is the Custom Extensions Module related to the other Modules?

The custom extensions module registers extensions used in your own custom applications to ensure they don't conflict with other PBX Modules.